.img {
    width: 350px;
    height: 350px;
    background: url('./imgs/1.jpg');
    position: relative;
}

.mask {
    width: 50px;
    height: 50px;
    background: yellow;
    opacity: .5;
    position: absolute;
    left:0px;
    top: 0px;
    display: none;
}

.enlarge {
    width: 100px;
    height: 100px;
    background: url('./imgs/1.big.jpg');
    background-position: 0px 0px;
    display: none;
}